In recent years, electric vehicles have been proposed and put to practical use for low-emission transportation. Among them are series hybrid vehicles. A typical series hybrid vehicle uses an internal combustion (IC) engine to make a generator to create electric power to be delivered to a traction motor. The traction motor powers the vehicle. The series hybrid electric vehicle is different from the other hybrid electric vehicles in that the engine power is delivered to the generator only for electric power generation and it is not delivered to the vehicle drive wheels for traction.
In Patent Literature 1 later mentioned, techniques are disclosed as follows: For the series hybrid vehicle, in order to prevent deterioration of the battery unit to used as the power supply for the traction motor, an allowable level or upper limit is set for electric power or current available from the battery unit, called, an allowable discharge capacity or current level, based on such conditions as the battery temperature and the battery voltage. In cold regions, the driver power demand may not be met because the allowable discharge capacity level is lowered due to a drop in the battery temperature.
In view of the above-mentioned driver's discomfort, the techniques disclosed in Patent Literature 1 attempt to meet the driver power demand by initiating the engine starting procedure upon or immediately after actual current level needed for powering the vehicle exceeds the allowable current level to cause the generator to create electric power and delivering the created power to the traction motor.